Understanding A Consistent Depth Of Field With Varying Focal Lengths

Understanding A Consistent Depth Of Field With Varying Focal Lengths


Copyright Matt Brandon

Matt Brandon of The Digital Trekker did such a great job of explaining how depth of field does not change with focal length (considering aperture stays the same) that I am not going to type it all up again, but instead, link directly to his post.

What Matt says in this post is fairly simple for some, but mind-bendingly difficult for others to conceptualize. We’re all wired differently but I think Matt’s way of explaining the concept works well. Basically he’s pointing out, through a clickable image, how depth of field does not change with varying focal lengths.

The important caveat here is that he kept the subject the same proportions in the field of view. You have seen this in Hollywood films and it is known as the Vertigo or Dolly Zoom effect because the camera is moved closer, or further, from the subject on a dolly to keep proportions the same while changing focal lengths. Such as shown here:

This can further be verified by playing with a depth of field calculator such as DOF Master.

If I go to the site and put in Canon 7D, 20mm lens, f/8 and a subject distance of 3′, I get a total depth of field of 2.31′.

If I change my focal length to 30mm, my depth of field shrinks to .92′. It also means the subject has become larger in the frame, filling more of it.

So if I want the subject to remain the same size in the frame, I need to physically move back. In testing, I can find that I now need to be 4.65′ away from the subject to give the same proportion in the frame. This also gives me a depth of field of 2.31′.

If you don’t believe me, take a look at Matt’s post and then try it yourself. One of the aspects of photography that I love the most is the ability to test and confirm things. There is no black magic in photography, it’s just all light and physics. And art, it’s art too. 🙂

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Peter West Carey leads photo tours and workshops in Nepal, Bhutan, Seattle, Portland, Los Angeles and beyond. He is also the creator of Photography Basics - A 43 Day Adventure & 40 Photography Experiments, web-based tutorials taking curious photographers on a fun ride through the basics of learning photography.

Some Older Comments

  • Allan October 23, 2012 06:21 pm

    Hi Edmund
    Thanks for your kind words.
    Most of my teaching is done face to face, however from my website (www.nzphotoworkshops.com) there is a link to a Facebook page where I occasional post photo tips. At some stage in the future I hope to put some of my lessons online. I’d even like to post a few of my lessons to this site.

  • edmund October 20, 2012 02:16 pm

    My previous inspiration was Scott Kelby, please let me know whether you have any books or on-line instruction. Your writing is fabulous!

  • Allan October 20, 2012 08:59 am

    @Edmund. Yes you have it exactly right. This is one of the big reasons for choosing different focal length lens. Most people will use a wide-angle lens to get a large expanse of the scene in, or a long lens to pick out some detail, but the way different lens affect perspective is where the real power of different lens lie (especially wide-angles).
    With a long lens, simular sized objects at different distances will appear to be the same size (sometimes called stacking, as the objects appear to be all stacked together).
    With a wide angle, the objects close to the camera appear large and the objects off in the distance become small (as if there was a whole lot of distance between the objects being photographed). Wide-angles aren’t easy to get your head around, but once you do, they become a very powerful tool. Most people never realise their full potential.
    Your idea of going out and experimenting is just the thing to do. It’s an exercise I set for my students, which they get a lot from.

  • edmund October 19, 2012 07:41 pm

    That's brilliant, Allan. I hadn't thought of it in terms of size of image in the viewfinder (although this is also affected by size of image on the sensor / film). However, the image (perspective) changes hugely with the choice of lens. If I photograph the same object, let's say a well known street, and keep a single lamp post the same size in the viewfinder then with a wideangle and a telephoto the scene is completely different. So the choice of lens also makes a difference because although the single object is the same size the other surroundings differ in relation to that object. Am I right or wrong here? Maybe I have never managed to carry out this experiment scientifically enough.

  • Allan October 19, 2012 12:23 pm

    @ Edmund, you’ve kind of missed the point and at the same time hit it exactly. You say “it’s not just a matter of focal length and aperture”. The point is that it’s not a matter of focal length at all. Provided the object stays the same size in the frame there are only two things that affect DOF: aperture, and as you have noticed, sensor size. Not Focal length as most books etc will tell you.
    Focal length and distance to subject have an effect, but they directly cancel each other out when we keep the object being photographed the same size.
    Think of it this way: if we zoom in or walk in DOF gets less. If we zoom out or walk out DOF becomes greater. If we zoom in and walk out, or zoom out and walk in then they cancel each other out.
    So when we are photographing an object (and we don’t want it’s size to change) the only ways we can change DOF is to change aperture or change cameras.
    Hope all this helps.

  • rex September 28, 2012 12:59 pm

    Very good post, made me think, thanks

  • Edmund September 28, 2012 11:09 am

    This whole DoF article is fine but ignores sensor size. On my wife's point and click camera, just about everything is in focus (regardless of the focal length of the zoom) because the sensor is tiny. On my 1930s 5X4 film camera I can take a portrait with just the eyes in focus and nothing else. In between comes a 35mm full frame sensor or an APS-C or a 4/3rds. I don't think that any article on DoF can ignore the fact that the smaller the sensor the greater the DoF, it's not just a matter of focal length and aperture, although these are important and in most beginner's photography manuals. So, Peter, please respond with the full story!

  • Fritz Bil September 28, 2012 10:55 am

    It would be interesting if there is a way to keep depth of field constant while zooming in or out. My camera (Canon 60D by the way) has Aperture Priority and Shutter Priority, but as far as I know there is no Depth of Field priority.

    Does anyone have a clue how to do this?

  • Max Imus September 28, 2012 03:27 am

    I've always wanted to know how they did that and now I do...sort of. Great Post.
    This is the nitty-gritty technical background one needs to master to really make that magic happen. Knowledge is Power!

    Thanks for sharing Peter and Matt.

  • Dylan September 27, 2012 03:26 pm

    This will always remind me of the Lion King.

  • Jai Catalano September 27, 2012 07:01 am

    Now this is one of the best posts. Not easy for everyone to grab but very well done.

  • Brian Fuller September 27, 2012 05:28 am

    Perfect explanation. Love the cube video.